This invention relates generally to a power transfer mechanism adapted to be incorporated in a multiple path drive system, for example a four-wheel drive vehicle. More particularly, it relates to a torque transfer case adapted to receive torque from a prime mover and to provide torque for one drive axle when two-wheel drive is adequate and to provide torque for both drive axles automatically when four-wheel drive is required. The torque transfer case incorporates a double-acting overrunning clutch for automatically engaging and disengaging the four-wheel drive function.
In recent years there have been many improvements in automotive drive trains, including improvements relating to the transfer of torque from a prime mover to drive axles. Where four-wheel drive systems are used, transfer cases have been developed which direct torque to one output to drive a fixed axle and through an overrunning clutch to another output for driving a steering axle. One such transfer case is disclosed in U.S. Pat. No. 3,993,152 issued Nov. 23, 1976, wherein an overrunning clutch automatically engages and disengages the four-wheel drive function. However, an excessive overrunning condition may cause undesirable clutch lockup, thus negating its automatic function. One attempt at a solution to this problem was the use of a speed-sensitive blocking structure, as dislcosed in U.S. Pat. No. 4,044,870 issued Aug. 30, 1977. It then was found that an excessive overrunning condition may obtain at speeds below the threshold speed of such a structure. Thus, there was still a need to provide an overrunning clutch which engages and disengages automatically, but which prevents undesirable engagement even at low speeds. This need is believed to have been met by a blocking structure which is direction-sensitive rather than speed-sensitive. Such a direction-sensitive blocking structure is disclosed in U.S. Pat. No. 4,047,601 issued Sept. 13, 1977 and 4,083,419 issued Apr. 11, 1978.
The direction-sensitive blocking structure includes a drag ring and a blocking element which develop opposing frictional forces continuously during operation. As a result, the operating characteristics of the mechanism are not such as to provide efficient energy utilization.